1. Field of the Invention
This invention relates to a digital protective relay apparatus to be used for protecting a power system, and more particularly to an apparatus of this type equipped with a function of correcting an error caused in an analog input circuit incorporated therein.
2. Description of the Related Art
A conventional digital protective relay apparatus will be described referring to FIG. 4.
In the digital protective relay apparatus, the various analog amounts of a power system employed in the apparatus, such as current flowing through the power supply line and bus line of the power system and voltage applied thereto, are input in parallel into an analog input circuit 1.
The analog input circuit 1 is constructed generally as shown in FIG. 5. Specifically, it has a plurality of input channels each comprising a transformer 2, a filter circuit 3, and a sample/hold circuit 4. The output terminal of each circuit 4 is connected to a multiplexer 5.
The voltage or current input to each input channel of the circuit 1 is supplied to a corresponding transformer 2 where it is subjected to level transform, and is then supplied to the filter circuit 3 where those high frequency components contained in it which are unnecessary for protection calculation are removed. Thereafter, an output signal from the filter circuit 3 is input to the sample/hold circuit 4, where it is sampled at predetermined intervals of time and the resultant sampling value is kept for a predetermined period of time.
The multiplexer 5 changes the outputs of the sample/hold circuits 4 from one to another, thereby selecting the analog values of the input channels.
The analog signal selected by the multiplexer 5 is input to an A/D converter 6, where it is converted to digital data signal. The digital data signal is input to a protective relay calculation circuit 7.
The circuit 7 performs predetermined protective relay calculation using the input digital data signal, and supplies an output circuit 8 with a trip trigger signal when the calculation result indicates occurrence of a fault in the power system. Upon receipt of the trip trigger signal from the calculation circuit 7, the output circuit 8 outputs a trip instruction to a breaker provided in the power system.
In the analog input circuit 1, there exist errors in the characteristics of an output with respect to an input (i.e., in the amplitude gain of the output and/or in the phase of the same) of the output relative to the input due to various constants of the structural elements therein. These errors should be eliminated since they adversely affect the protective function of the circuit.
To this end, in the conventional digital protective relay apparatus, the analog input circuit 1 incorporates a correction circuit 13 comprising an adjustable resistor 11 and an adjustable capacitor 12 and disposed to perform error-correction of digital data, as is shown in FIG. 6.
In this conventional apparatus, a calibration signal having a waveform similar to that of an analog amount of electricity is input to the analog input circuit 1 from the power system, and is detected after having passed the circuit 1. The variable resistor 11 and variable capacitor 12 are adjusted in accordance with a detection result.
Since the constant of each component of the analog input circuit 1 varies with the lapse of time, it is necessary to periodically adjust the correction circuit 13.
As described above, the conventional digital protective relay apparatus is disadvantageous in that its calibration procedure is complicated and hence requires much labor.